This invention relates to a semiconductor light emitting element, its manufacturing method and a light emitting device. More specifically, the invention relates to a light emitting element, its manufacturing method and a light emitting device in which the p-side electrode and the n-side electrode of a light emitting element using nitride compound semiconductors are formed on a common plane to obtain a high performance and high integration and to simplify the manufacturing and assembling process.
Semiconductor light emitting elements have advantageous features, namely, compactness, high reliability, high efficiency, and capability of emitting light in a wide wavelength range from the infrared range to the blue-violet range when appropriate materials are chosen. Making the best use of these features, they are widely used regardless of the industrial use or civil use.
Made below is a review of light emitting elements using nitride compound semiconductors among such semiconductor light emitting elements. Throughout the specification, the nitride compound semiconductors pertain to any semiconductors with all varieties of mole fractions x and y from zero to 1 in the chemical formula In.sub.x Al.sub.y Ga.sub.1-x-y N 0.ltoreq.x.ltoreq.1 , 0.ltoreq.y.ltoreq.1, x+y=1). For example, GaN (x=0, y=0) is also one of nitride compound semiconductors.
Nitride compound semiconductors are III-V compound semiconductors of direct transition type, whose band gaps can be varied from 1.89 to 6.2 eV by controlling their mole fractions x and y, and are remarked as hopeful materials of LED (light emitting diodes) and semiconductor lasers. Especially, if they successfully emit highly luminous light in the blue wavelength range, it is possible to double the recording capacity of various optical disks and to provide full color displays. From these expectations, blue light emitting elements using In.sub.x Al.sub.y Ga.sub.1-x-y N semiconductors have been used to improve initial characteristics and reliability.
Referential documents disclosing conventional blue light emitting elements using nitride compound semiconductors involve Jpn. J. Appl. Phys., 28(1989) p.L2 112, Jpn. J. Appl. Phys., 32(1993) p. L8, and Japanese Patent Laid-Open Publication 5-291621.
FIG. 8 is a schematic cross-sectional view of a conventional blue light emitting element.
Its construction is roughly explained below.
The light emitting element 100 has a multi-layered structure stacking semiconductors on a sapphire substrate 102. More specifically, stacked on the sapphire substrate 102 are a buffer layer 104, n-type contact layer 106, active layer 108, and p-type contact layer 110 in this order.
The stacked structure is partly removed by etching to form a step where the n-type contact layer 106 is partly exposed. The n-side electrode 120 is formed on the exposed surface of the n-type contact layer 106, and the p-side electrode 130 is formed on the p-type contact layer 110. A reason of etching the structure to the n-type contact layer 106 to form the n-side electrode lies in that the sapphire substrate 102 has an electrically insulating property.
The buffer layer 104 may be made of GaN, for example. The n-type contact layer 106 is an n-type semiconductor layer having a high carrier concentration enough to ensure ohmic contact with the n-side electrode 120, and may be made of, for example, AlGaN doped with silicon (Si). The active layer 108 is a semiconductor layer where electric charges injected as a current into the light emitting element recombine and emit light. Usable as its material is, for example, In.sub.x Al.sub.y Ga.sub.1-x-y N doped with zinc (Zn). The p-type contact layer 110 is a p-type semiconductor layer having a high carrier concentration to ensure its ohmic contact with the p-side electrode, and may be made of, for example, AlGaN doped with magnesium (Mg).
When a current is injected to the light emitting element, light in the blue wavelength range is emitted in the active layer 108 having luminescent centers in zinc (Zn).
However, since the conventional blue light emitting element shown in FIG. 8 makes the n-side electrode 120 on the surface of the n-type contact layer 106, it needs the process of etching the structure to the depth of the n-type contact layer 106. Moreover, the n-side electrode 120 must be made on the bottom surface of a step formed by the etching. That is, it is necessary to stack materials of the n-side electrode 120 on the bottom surface of the step and to pattern it appropriately. This process is not easy, and are apt to decrease the yield.
Another problem of the conventional element lies in that the light emitting element 100 has a step and has the n-side electrode 120 and the p-side electrode in different levels, which results in requiring positional adjustment to respective levels of the electrodes and therefore complicating the process. In the step of wire bonding to the n-side electrode 120, a bonding tool for supplying the wire may bump against the side surface of the step (shown at S in FIG. 8). To avoid this, the bottom surface of the step must be enlarged. It cause another problem, namely, an increase of the element size and the manufacturing cost.
Conventional light emitting elements having the n-side electrode and the p-side electrode in different levels, and not on a common plane, made so-called flip-chip mounting difficult. Another problem, therefore, is that light emitting elements could not be used to various applications relying on flip-chip mounting and that it was difficult to provide improvements of electric, optical characteristics and reduction of the packaging size.
All these problems with conventional techniques as explained above are caused by the step-containing structure of conventional gallium nitride light emitting elements.